- Email: [email protected]
Growth of oxalate crystals on films of acrylate polymers
ELSEVIER
MaterialsScienceand Engineering C 4 ( 1996) 133-137
Growth of oxalate crystals on films of acrylate polymers
’
J. Burdon, M. Oner, P. Calvert Arizona Materials Labs., University of Arizona, Tucson, AZ, USA
Abstract Cast films of a series of acr,ylic copolymers and homopolymers were immersed in a crystallizing solutions of calcium oxalate which also contained a growth inhibitor. IHydroxylated polyacrylates, but not carboxylated polyacrylates, were found to form films of calcium oxalate. Microscopic observations of the time sequence for film formation suggested that crystals are nucleated inside the polymer film and then grow up through the surface. Keywords:
Oxalate crystals; Acrylate polymers
1. Introduction
Biological mineralization demonstrates the possibility of growing inorganic minerals locally on or in polymer substrates [ 11. This ability could usefully be transferred to the formation of patterned films of active inorganic compounds for application as devices, such as chemical, stress or light sensors. Some synthetic ex,amples of such patterned growth of minerals from aqueous solutions have been developed recently [ 21. The prevailing belief is that the growth of inorganic crystals on polymer substrates will be determined by epitaxial nucleation, where there is a close match of two-dimensional spacing between ions on the crystal surface and charged groups on the polymer. Given the disordered nature of most polymer surfaces, it is difficult to see how true epitaxy can occur. Mann and co-workers have emphasized the importance of a match in local geometry between binding groups on the polymer and ions in the crystal surface [ 31. A number of other mechanisms ,may play a role in controlling the site of mineralization [4]. These include local generation of precipitating ions, adsorption of nuclei from solution onto the polymer surface, exclusion of growth inhibitors from the surface region and nucleation within a swollen polymer due to reduced surface energies. ’ Paper presentedat SymposiumS: Biomolecularaad BiomimeticMaterials MRS Fall Meeting, Boston, IJSA, November 2%December 2,1994. This work relates to Department of Navy Grant NOO014-95-1-0131 issued by the Office of Navel Research. The United States Government has a royalty-free license throughout the world in an copyrightable material contained herein. 092%4931/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIO928-4931(95)00135-X
Even if surface nucleation effects are important, they are not the only component of the problem. Nucleation and growth in the solution must be sufficiently slow so that the surface process dominates. This usually requires low supersaturations and low growth rates, at which point diffusion of ions to the surface can be limiting and mixing processes are important. This paper describes the growth of calcium oxalate crystals on a range of acrylate polymers and copolymers. The mineralizing solutions were adjusted to give slow growth rates and had polyacrylic acid in solution as a growth inhibitor. The volume of the polymer was very small to avoid producing any significant change in the solution equilibrium. This study builds on previous work on inhibition of crystal growth of calcium oxalates by soluble acrylic polymers [ 51. An earlier paper also reported calcium oxalate mineralization of crosslinked gels of copolymer (butylmethacrylate-methacrylic acid) [6].
2. Experimental 2.1. Polymer synthesis and$lm preparation Copolymers and homopolymers were synthesized in 100 ml 3-neck round-bottomed flasks. Reactions were performed in cyclohexanone at 65 “C with 1 ml of stock azoisobutyronitrile (AIBN) initiator solution (0.5 g AIBN in 20 ml cyclohexanone) . Total monomer volume was 10 ml. The reaction was stirred for 24 h. Polymer films were formed by casting thin films on microscope slides, air drying for 4 h, followed by vacuum-oven dry at 55 “C for 24 h.
134
J, Burdon et al. /Materials
Science and Engineering
2.2. Copolymer identijcation and analysis Due to the lack of solubility of the copolymers in common solvents for GPC and NMR analysis, only the copolymer based on 3-hydroxypropylacrylate was analyzed. The 3hydroxypropylacrylate (HPA) copolymer was soluble in THF making it suitable for molecular weight analysis. The HPA copolymer was not soluble in any solvents for NMR analysis however. GPC analysis using a refractive index detector gave a single peak with polystyrene-equivalent M, of 29500 and M, of 38500. UV detection gave similar values (M, 26000 and M, 37000) implying that there was no strong segregation of the comonomers. 2.3. Crystal growth procedures Crystal growth investigations were performed in supersaturated solutions of calcium oxalate at pH 10.3-10.4. Supersaturated solutions were prepared by mixing 4.5 X lop4 M CaCl, solution with 4.5 X 10e4 M Na2C204 solution containing 50 ppm polyacrylic acid inhibitor, pH adjusted to 10.310.4. The two solutions were mixed over a period of approximately 30 s, and time zero was taken as the time to complete addition. The use of an inhibitor in solution has previously been shown to promote mineralization on surfaces [ 4,7,8]. A more appropriate technique was developed to study single crystal nucleation and growth on the polymer films. Micro-glass bulbs were produced from heat-sealed capillary tubes. The micro-bulbs were then dip-coated in polymer solution. Post-treatment of the samples was identical to conventional large film samples as described in a previous section. By using multiple samples (typically 10-20) and removing samples at time intervals of as small as 30 s, it was possible to observe crystal growth as a function of time. Samples removed from the growth solution were quickly rinsed in deionized water and air dried. The small size of the samples allowed a complete experimental run of samples to be loaded onto a single SEM sample mount for quick analysis. Field emission scanning electron microscopy analysis was used to identify early growth forms. Light Scattering was carried out using a He-Ne laser split into two beams one of which traversed the solution and the other also intercepted the polymer film. The intensity of light transmitted in the beam was measured in both cases and subtracted from the initial intensity to give a measure of scattering.
3. Results 3.1. Mineralization on swellable and insoluble copolymers A series of copolymers were prepared, selected for a combination of water-soluble component with sufficient waterinsoluble monomer to render the copolymer insoluble. It was
Table 1 Mineralization
C4 (1996)
133-137
of copolymer
films
Copolymer 10% 10% 50% 20% 30% 20% 40% 20% 30% 20% 80% 20% 50% 50% 80% 60% 80% 80% 50% 80% 80% 80%
MAA, 90% THPPMA THPFMA, 90% Butyl MA MAA, 50% THFFMA MAA, 80% THPPMA MAA, 70% THWMA MAA, 80% Butyl MA MAA, 60% THPPMA THPPMA, 80% Butyl MA MAA, 70% Butyl MA HEMA, 80% THPPMA HEMA, 20% THFFMA MAA, 80% Ethoxy Ethoxy, 50% Butyl MA Ethoxy, 50% THPPMA HEMA, 20% MAA HEMA, 40% MAA HEMA, 20% Butyl MA HEMA, 20% Ethoxy MAA, 50% Butyl MA HBA, 20% THFFMA HPMA, 20% THPPMA HPA, 20% THPPMA
Result (No entry: no effect)
Dissolves Cloudy in DI water
Some surface crystallization Surface crystallization
Insoluble, intractable polymer Insoluble, intractable polymer
Dissolves Surface crystallization Surface crystallization Surface crystalkation
MAA: Methacrylic acid, Butyl MA: Butylmethacrylate, THPPMA: Tetrahydrofurfurylmethacrylate, Ethoxy: 2-Ethoxyethylmethacrylate, HEMA: 2Hydroxyethylmethacrylate, HPA: 3-Hydroxypropylacrylate, HPMA: 3-Hydroxypropylmethacrylate, HBA: 4-Hydroxybutylacrylate.
expected that this strategy would provide films with a very hydrophilic surface and a high level of water-swelling. Based on previous work on inhibition by copolymers, it was expected that acrylic acid copolymers would be most effective [ 51. Table 1 shows the results of these experiments. Scanning electron microscopy showed differing crystal morphologies for each of the four active copolymers with 20% THFFMA: 2-hydroxyethyl methacrylate 3-hydroxypropyl methacrylate 4-hydroxybutyl acrylate 3-hydroxypropyl acrylate
rosettes mushrooms stars agglomerates
During routine repeat crystal growth experiments on new batches of HEMA based copolymer, no crystal growth promotion was observed, contradicting the results obtained for the original batch of HEMA copolymer. It was suspected that this effect may have involved the extent of the reaction because the original HEMA copolymer solution was of a lower viscosity than subsequent batches. This effect was investigated by synthesizing a further batch of HEMA copolymer, and removing small quantities of reaction mixture every hour for up to 50 h to prepare films for subsequent crystal growth analysis. Samples cast from reaction mixture up to 7 h showed no polymer film formation and were discarded. Samples from 8-24 h showed crystal growth promotion, whereas 46 h and higher samples showed no crystal growth. Differing mono-
J. Burdon et al. /Materials Science and Engineering C4 (1996) 133-137
Time
( mins
135
)
Fig. 1. Light scattering from solution and from sample surface for (a) HEMA-THFFMA copolymer and (b) HBMA-THFFMA copolymer.
mer reactivities will lead to a changing polymer composition during extended polymerizations. Increasing the solution viscosity will also lead to an increasing polymer molecular weight [ 91. Fig. 1 shows light scattering from the film and solution during crystallization on HEMA copolymer and 4-hydroxybutylacrylate copolymer. In the case of the HEMA based copolymer sample and solution scattering begin to show differences at around 40 min of exposure, whereas with the 4hydroxybutylacrylate based copolymer, the divergence of the sample and solution scattering is not evident until around 112 min of exposure time. Light scattering provided a convenient method to analyze crystal growth on polymer films. However, it became evident from SEM analysis of the film samples that it was difficult to distinguish between actual single crystal growth which is occurring solely on and/or in the polymer surface and crystal overgrowth. SEM analysis of films used in light scattering experiments revealed that in most cases the light scattering measurements were following crystal over-growth rather than the early stages of crystal formation mediated by the surrounding polymer. In this light, experiments were carried out by sequential sampling of polymer film on capillaries, taken from crystallizing solutions. The HEMA copolymer showed crystal growth at short times (l-5 min), and revealed sporadic growth of nanometer-sized particles. This is in contrast to the
Fig. 2. Micrographs of calcium oxalate growing on HEMA-THFFMA ymer after (a) 40 min and (b) 85 min.
copol-
light scattering which indicated that crystals were present after 30-40 min of exposure. Experiments with HBA based copolymer revealed very small scale crystals (sub- 100 nm) at up to 5 min, 200 nm at 20 min, and larger scale crystals at 30 min. The light scattering data showed substantial crystal growth at 112 min and above. A full analysis of the growth of calcium oxalate on HEMAbased copolymer was performed using micro-samples over a time range of 10-90 min. Micrographs from these experiments are shown in Fig. 2, showing 40 and 85 min exposure samples. The samples had crystal growth visible to the eye after 30 min, which is in agreement with the laser light scattering data. 3.2. Mineralization on homopolymers Homopolymers of HEA, HPA and HBA were synthesized and investigated under identical conditions for crystal growth promotion. It was found that HEA, HPA and HBA homopolymers also showed substantial crystal growth promotion. The HEA homopolymer showed no evidence of crystal growth at intervals of 6,12,18,24,30 and 36 min, but showed crystals at 42 min and above. Homopolymer HBA showed a few sporadic crystals at 6 min and was covered at 12 min.
136
J. Burden et al. /Materials Science and Engineering C4 (1996) 133-137
The sporadic nature of the crystal growth was of concern and was investigated further. It was noticed also that the crystal growth occurred more at the polymer interface where the polymer layer began (at the bottom of the sample, furthest from the tip). To ascertain whether this might have been a surface effect due solely to the shape of the sample tip, new micro-flat samples were produced, which would be able to mimic the large, flat films originally produced on microslides. The micro-flat samples also revealed crystal growth at different densities on the sample, with crystal growth beginning from the edge and radiating towards the center. The following effects were evident: The crystal number density increased with time. The average crystal size increased with time. The crystal size was uniform on each sample, independent of location in relation to the edge. Samples taken at 3, 6, 9 and 12 min showed that crystal growth progressed in from the edge of the polymer layer at a rate of about 2 pm/min. Fig. 3 shows the edge of the polymer layer on the glass substrate exposed for 9 min. Calcium oxalate growth is evident with high concentrations at the edge of the polymer layer. Crystal growth moves towards the center of the film with increasing exposure time. The crystal length increased with exposure time but it was independent of position on the sample even though crystal density was initially higher near the edges. Fig. 4 shows crystals on a sample exposed for 20 min. The growth data are summarized in Table 2. The row indicating exposure time of 12 (zero + 20) refers to data taken from a 12 min exposure sample. For this sample however, time zero was taken as 20 min into the experiment and thus the sample was placed in the solution after 20 min, and removed after 32 min. The crystal length is comparable with the initial 12 min exposure sample which was immersed in the solution at time zero. This indicates that the growth of calcium oxalate on HBA homopolymer proceeds via a nucleation and growth mechanism, rather than by the capture and growth of calcium oxalate crystals formed in solution. If growth occurred by capture and growth of calcium oxalate
Fig. 4. Crystals grown on HBA homopolymer
for 20 min.
‘able 2 kystal size and edge distances for CaCZ04 growth on HBA homopolymer Exposure time
Crystal length
(min)
(km)
Distance from polymer edge
3 6 9 12 12(zero+20) 20 30
0.30 0.47 0.60 0.68 0.65 0.94 > 1.00
20 I*rn 150 km 1.5 mm center 150 pm 150 pm 150 pm
.
Fig. 5. Increase of crystal size with time on HBA homopolymer.
crystals formed in solution, then the crystal length should be greater than 1 pm, as indicated by the longer-term exposure 20 min and 30 min sample data. As seen from Fig. 5, growth is roughly linear with time.
4. Discussion
Fig. 3. Crystal growth near edge of HBA homopolymer
film after 9 min.
The comparison of different copolymers shows a strong effect of polymer structure on mineralization. The effect of polymerization time for the HEMA copolymer particularly illustrates this. Since these polymers are all amorphous and the copolymers have a random sequences there is no reason to anticipate any regular structures on the surface.
J. Burdon et al. /Materials Science and Engineering C4 (1996) 133-137
Hydroxylated polymers, t.hat are highly swellable in water but not actually soluble, seem to be most efficient. We had expected that copolymers containing c&boxylic acid groups would be effective but they do not seem to be. One interpretation is that the swollen polymer excludes the polyacrylic acid growth inhibitor, due to osmotic effects. A second view is that the hydroxylated pol:vmer provides the right amount of binding for the calcium ions to lower the crystal surface energy, without binding strongly to the surface and inhibiting growth. The use of a polymeric growth inhibitor has certainly been shown to be effective with other similar polymer films [ 7,8]. Zhang and Gonsalves interpret this effect in terms of a modification of the surface chemistry of the polymer film. We considered the possibility that nucleation occurred in the solution and crystals adsorbed to the surface. The effect of allowing the solution to stand and then adding the film is that the crystals grow as if put into a freshly mixed solution. Hence nucleation in the polymer does seem to be the predominant effect. It is clear from the micrographs that crystals do grow on the polymer surface but that the initial growth occurs within or partly within the surface and that crystals later break through. The increase of crystal size with time reflects a growth rate of 40 nm/min, which is reasonable. The relatively uniform crystal size suggests early nucleation followed by growth rather than continuing nucleation. This is explicable in terms of solute depletion in the polymer film or near the surface, once a substantial number of crystals have started to grow. Since nucleation is far more sensitive to supersaturation, it will be suppressed while growth continues [ 4,101. The moving growth front seen in Fig. 3 contradicts these ideas since it suggests that areas further from the edge nucleate later. In fact it moves slowly in from the film edge and then travels rapidly across the remaining film. We believe this moving front reflects nucleation on the glass-polymer interface or within the films, coupled with thinner films near the edge. The strong contrast seen at low magnification in the SEM is due to crystals which have broken through the surface, which happens earlier when the film is thinner. Unfortunately the very small sample size and bulb geometry, needed for crystallization under conditions of constant solution composition, do not lend themselves to sectioning through the films. Some evidence for the thickness effect is that the shape of the white band near the edge does apparently reflect changes in film topography. Similar films cast from solution onto microscope slides were weighed and found to have a thickness of a few microns.
137
Given that crystal growth originates within the film, the nucleation site may be in the polymer itself, on adventitious dust particles in the film or at the polymer-glass interface. These experiments were done with small areas of very thin films so that minimal change in the solution conditions occurs. There must nonetheless be some local change as the polymer equilibrates with the solution. Different results would be expected using large amounts of thick films or on gels which have a pH or other ion concentrations different from those in the solution.
5. Conclusions A series of hydroxylated acrylate polymers have been shown to promote surface mineralization by calcium oxalate from aqueous solution. Crystals initially form within the polymer but break through onto the surface as they grow. The effect is very sensitive to polymer structure and apparently reflects some combination of swellability and calcium ion binding.
Acknowledgements We would like to thank the Army Research support of this work.
Office for
References [I] P.D. Calvert and S. Mann, Journal ofMaterials Science, 23 ( 1988) 3801. [2] P. Rieke, B. Tarasevich, L. Wood, M. Engelhard, D. Baer, G. Fryxell, C. John, D. Laken and M. Jaehnig, Longmuir, 10 (1994) 619.
[3] S. Mann, D.D. Archibald, J.M. Didymus, T. Douglas, B.R. Heywood, F.C. Meldrum and N.J.Reeves, Science, 261 (1993) 1286. [4] P. Calve& Materials Research Society Symposium Proceedings, 330 (1994) 79. [5] M. Oner and P. Calvert, Materials Science and Engineering C, 2 (1994) 93. [ 61 P. Calve& M. Oner, J. Burdon, P. Rieke and K. Farmer, in SPIE: Smart Structures andMaterials; Albuquerque, NM, 1993, p. 354. [ 71 SK. Zhang and K.E. Gonsalves, Journal of Applied Polymer Science, 56 (1995) 687. [8] M.A. Crenshaw, A. Linde and A. Lussi, in LA. Aksay, G.L. McVay and J.F. Wager (eds.), Atomic and Molecular Processing of Electronicand Ceramic Materials; Materials Research Society,
Pittsburgh, PA, 1987, p. 99. [9] G.G. Odian, Principles of Polymerization, 3rd edn., Wiley, New York, 1991. [ 101 P.D. Calvert, Materials Research Society Symposium Proceedings, 73 (1986) 79.
MaterialsScienceand Engineering C 4 ( 1996) 133-137
Growth of oxalate crystals on films of acrylate polymers
’
J. Burdon, M. Oner, P. Calvert Arizona Materials Labs., University of Arizona, Tucson, AZ, USA
Abstract Cast films of a series of acr,ylic copolymers and homopolymers were immersed in a crystallizing solutions of calcium oxalate which also contained a growth inhibitor. IHydroxylated polyacrylates, but not carboxylated polyacrylates, were found to form films of calcium oxalate. Microscopic observations of the time sequence for film formation suggested that crystals are nucleated inside the polymer film and then grow up through the surface. Keywords:
Oxalate crystals; Acrylate polymers
1. Introduction
Biological mineralization demonstrates the possibility of growing inorganic minerals locally on or in polymer substrates [ 11. This ability could usefully be transferred to the formation of patterned films of active inorganic compounds for application as devices, such as chemical, stress or light sensors. Some synthetic ex,amples of such patterned growth of minerals from aqueous solutions have been developed recently [ 21. The prevailing belief is that the growth of inorganic crystals on polymer substrates will be determined by epitaxial nucleation, where there is a close match of two-dimensional spacing between ions on the crystal surface and charged groups on the polymer. Given the disordered nature of most polymer surfaces, it is difficult to see how true epitaxy can occur. Mann and co-workers have emphasized the importance of a match in local geometry between binding groups on the polymer and ions in the crystal surface [ 31. A number of other mechanisms ,may play a role in controlling the site of mineralization [4]. These include local generation of precipitating ions, adsorption of nuclei from solution onto the polymer surface, exclusion of growth inhibitors from the surface region and nucleation within a swollen polymer due to reduced surface energies. ’ Paper presentedat SymposiumS: Biomolecularaad BiomimeticMaterials MRS Fall Meeting, Boston, IJSA, November 2%December 2,1994. This work relates to Department of Navy Grant NOO014-95-1-0131 issued by the Office of Navel Research. The United States Government has a royalty-free license throughout the world in an copyrightable material contained herein. 092%4931/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIO928-4931(95)00135-X
Even if surface nucleation effects are important, they are not the only component of the problem. Nucleation and growth in the solution must be sufficiently slow so that the surface process dominates. This usually requires low supersaturations and low growth rates, at which point diffusion of ions to the surface can be limiting and mixing processes are important. This paper describes the growth of calcium oxalate crystals on a range of acrylate polymers and copolymers. The mineralizing solutions were adjusted to give slow growth rates and had polyacrylic acid in solution as a growth inhibitor. The volume of the polymer was very small to avoid producing any significant change in the solution equilibrium. This study builds on previous work on inhibition of crystal growth of calcium oxalates by soluble acrylic polymers [ 51. An earlier paper also reported calcium oxalate mineralization of crosslinked gels of copolymer (butylmethacrylate-methacrylic acid) [6].
2. Experimental 2.1. Polymer synthesis and$lm preparation Copolymers and homopolymers were synthesized in 100 ml 3-neck round-bottomed flasks. Reactions were performed in cyclohexanone at 65 “C with 1 ml of stock azoisobutyronitrile (AIBN) initiator solution (0.5 g AIBN in 20 ml cyclohexanone) . Total monomer volume was 10 ml. The reaction was stirred for 24 h. Polymer films were formed by casting thin films on microscope slides, air drying for 4 h, followed by vacuum-oven dry at 55 “C for 24 h.
134
J, Burdon et al. /Materials
Science and Engineering
2.2. Copolymer identijcation and analysis Due to the lack of solubility of the copolymers in common solvents for GPC and NMR analysis, only the copolymer based on 3-hydroxypropylacrylate was analyzed. The 3hydroxypropylacrylate (HPA) copolymer was soluble in THF making it suitable for molecular weight analysis. The HPA copolymer was not soluble in any solvents for NMR analysis however. GPC analysis using a refractive index detector gave a single peak with polystyrene-equivalent M, of 29500 and M, of 38500. UV detection gave similar values (M, 26000 and M, 37000) implying that there was no strong segregation of the comonomers. 2.3. Crystal growth procedures Crystal growth investigations were performed in supersaturated solutions of calcium oxalate at pH 10.3-10.4. Supersaturated solutions were prepared by mixing 4.5 X lop4 M CaCl, solution with 4.5 X 10e4 M Na2C204 solution containing 50 ppm polyacrylic acid inhibitor, pH adjusted to 10.310.4. The two solutions were mixed over a period of approximately 30 s, and time zero was taken as the time to complete addition. The use of an inhibitor in solution has previously been shown to promote mineralization on surfaces [ 4,7,8]. A more appropriate technique was developed to study single crystal nucleation and growth on the polymer films. Micro-glass bulbs were produced from heat-sealed capillary tubes. The micro-bulbs were then dip-coated in polymer solution. Post-treatment of the samples was identical to conventional large film samples as described in a previous section. By using multiple samples (typically 10-20) and removing samples at time intervals of as small as 30 s, it was possible to observe crystal growth as a function of time. Samples removed from the growth solution were quickly rinsed in deionized water and air dried. The small size of the samples allowed a complete experimental run of samples to be loaded onto a single SEM sample mount for quick analysis. Field emission scanning electron microscopy analysis was used to identify early growth forms. Light Scattering was carried out using a He-Ne laser split into two beams one of which traversed the solution and the other also intercepted the polymer film. The intensity of light transmitted in the beam was measured in both cases and subtracted from the initial intensity to give a measure of scattering.
3. Results 3.1. Mineralization on swellable and insoluble copolymers A series of copolymers were prepared, selected for a combination of water-soluble component with sufficient waterinsoluble monomer to render the copolymer insoluble. It was
Table 1 Mineralization
C4 (1996)
133-137
of copolymer
films
Copolymer 10% 10% 50% 20% 30% 20% 40% 20% 30% 20% 80% 20% 50% 50% 80% 60% 80% 80% 50% 80% 80% 80%
MAA, 90% THPPMA THPFMA, 90% Butyl MA MAA, 50% THFFMA MAA, 80% THPPMA MAA, 70% THWMA MAA, 80% Butyl MA MAA, 60% THPPMA THPPMA, 80% Butyl MA MAA, 70% Butyl MA HEMA, 80% THPPMA HEMA, 20% THFFMA MAA, 80% Ethoxy Ethoxy, 50% Butyl MA Ethoxy, 50% THPPMA HEMA, 20% MAA HEMA, 40% MAA HEMA, 20% Butyl MA HEMA, 20% Ethoxy MAA, 50% Butyl MA HBA, 20% THFFMA HPMA, 20% THPPMA HPA, 20% THPPMA
Result (No entry: no effect)
Dissolves Cloudy in DI water
Some surface crystallization Surface crystallization
Insoluble, intractable polymer Insoluble, intractable polymer
Dissolves Surface crystallization Surface crystallization Surface crystalkation
MAA: Methacrylic acid, Butyl MA: Butylmethacrylate, THPPMA: Tetrahydrofurfurylmethacrylate, Ethoxy: 2-Ethoxyethylmethacrylate, HEMA: 2Hydroxyethylmethacrylate, HPA: 3-Hydroxypropylacrylate, HPMA: 3-Hydroxypropylmethacrylate, HBA: 4-Hydroxybutylacrylate.
expected that this strategy would provide films with a very hydrophilic surface and a high level of water-swelling. Based on previous work on inhibition by copolymers, it was expected that acrylic acid copolymers would be most effective [ 51. Table 1 shows the results of these experiments. Scanning electron microscopy showed differing crystal morphologies for each of the four active copolymers with 20% THFFMA: 2-hydroxyethyl methacrylate 3-hydroxypropyl methacrylate 4-hydroxybutyl acrylate 3-hydroxypropyl acrylate
rosettes mushrooms stars agglomerates
During routine repeat crystal growth experiments on new batches of HEMA based copolymer, no crystal growth promotion was observed, contradicting the results obtained for the original batch of HEMA copolymer. It was suspected that this effect may have involved the extent of the reaction because the original HEMA copolymer solution was of a lower viscosity than subsequent batches. This effect was investigated by synthesizing a further batch of HEMA copolymer, and removing small quantities of reaction mixture every hour for up to 50 h to prepare films for subsequent crystal growth analysis. Samples cast from reaction mixture up to 7 h showed no polymer film formation and were discarded. Samples from 8-24 h showed crystal growth promotion, whereas 46 h and higher samples showed no crystal growth. Differing mono-
J. Burdon et al. /Materials Science and Engineering C4 (1996) 133-137
Time
( mins
135
)
Fig. 1. Light scattering from solution and from sample surface for (a) HEMA-THFFMA copolymer and (b) HBMA-THFFMA copolymer.
mer reactivities will lead to a changing polymer composition during extended polymerizations. Increasing the solution viscosity will also lead to an increasing polymer molecular weight [ 91. Fig. 1 shows light scattering from the film and solution during crystallization on HEMA copolymer and 4-hydroxybutylacrylate copolymer. In the case of the HEMA based copolymer sample and solution scattering begin to show differences at around 40 min of exposure, whereas with the 4hydroxybutylacrylate based copolymer, the divergence of the sample and solution scattering is not evident until around 112 min of exposure time. Light scattering provided a convenient method to analyze crystal growth on polymer films. However, it became evident from SEM analysis of the film samples that it was difficult to distinguish between actual single crystal growth which is occurring solely on and/or in the polymer surface and crystal overgrowth. SEM analysis of films used in light scattering experiments revealed that in most cases the light scattering measurements were following crystal over-growth rather than the early stages of crystal formation mediated by the surrounding polymer. In this light, experiments were carried out by sequential sampling of polymer film on capillaries, taken from crystallizing solutions. The HEMA copolymer showed crystal growth at short times (l-5 min), and revealed sporadic growth of nanometer-sized particles. This is in contrast to the
Fig. 2. Micrographs of calcium oxalate growing on HEMA-THFFMA ymer after (a) 40 min and (b) 85 min.
copol-
light scattering which indicated that crystals were present after 30-40 min of exposure. Experiments with HBA based copolymer revealed very small scale crystals (sub- 100 nm) at up to 5 min, 200 nm at 20 min, and larger scale crystals at 30 min. The light scattering data showed substantial crystal growth at 112 min and above. A full analysis of the growth of calcium oxalate on HEMAbased copolymer was performed using micro-samples over a time range of 10-90 min. Micrographs from these experiments are shown in Fig. 2, showing 40 and 85 min exposure samples. The samples had crystal growth visible to the eye after 30 min, which is in agreement with the laser light scattering data. 3.2. Mineralization on homopolymers Homopolymers of HEA, HPA and HBA were synthesized and investigated under identical conditions for crystal growth promotion. It was found that HEA, HPA and HBA homopolymers also showed substantial crystal growth promotion. The HEA homopolymer showed no evidence of crystal growth at intervals of 6,12,18,24,30 and 36 min, but showed crystals at 42 min and above. Homopolymer HBA showed a few sporadic crystals at 6 min and was covered at 12 min.
136
J. Burden et al. /Materials Science and Engineering C4 (1996) 133-137
The sporadic nature of the crystal growth was of concern and was investigated further. It was noticed also that the crystal growth occurred more at the polymer interface where the polymer layer began (at the bottom of the sample, furthest from the tip). To ascertain whether this might have been a surface effect due solely to the shape of the sample tip, new micro-flat samples were produced, which would be able to mimic the large, flat films originally produced on microslides. The micro-flat samples also revealed crystal growth at different densities on the sample, with crystal growth beginning from the edge and radiating towards the center. The following effects were evident: The crystal number density increased with time. The average crystal size increased with time. The crystal size was uniform on each sample, independent of location in relation to the edge. Samples taken at 3, 6, 9 and 12 min showed that crystal growth progressed in from the edge of the polymer layer at a rate of about 2 pm/min. Fig. 3 shows the edge of the polymer layer on the glass substrate exposed for 9 min. Calcium oxalate growth is evident with high concentrations at the edge of the polymer layer. Crystal growth moves towards the center of the film with increasing exposure time. The crystal length increased with exposure time but it was independent of position on the sample even though crystal density was initially higher near the edges. Fig. 4 shows crystals on a sample exposed for 20 min. The growth data are summarized in Table 2. The row indicating exposure time of 12 (zero + 20) refers to data taken from a 12 min exposure sample. For this sample however, time zero was taken as 20 min into the experiment and thus the sample was placed in the solution after 20 min, and removed after 32 min. The crystal length is comparable with the initial 12 min exposure sample which was immersed in the solution at time zero. This indicates that the growth of calcium oxalate on HBA homopolymer proceeds via a nucleation and growth mechanism, rather than by the capture and growth of calcium oxalate crystals formed in solution. If growth occurred by capture and growth of calcium oxalate
Fig. 4. Crystals grown on HBA homopolymer
for 20 min.
‘able 2 kystal size and edge distances for CaCZ04 growth on HBA homopolymer Exposure time
Crystal length
(min)
(km)
Distance from polymer edge
3 6 9 12 12(zero+20) 20 30
0.30 0.47 0.60 0.68 0.65 0.94 > 1.00
20 I*rn 150 km 1.5 mm center 150 pm 150 pm 150 pm
.
Fig. 5. Increase of crystal size with time on HBA homopolymer.
crystals formed in solution, then the crystal length should be greater than 1 pm, as indicated by the longer-term exposure 20 min and 30 min sample data. As seen from Fig. 5, growth is roughly linear with time.
4. Discussion
Fig. 3. Crystal growth near edge of HBA homopolymer
film after 9 min.
The comparison of different copolymers shows a strong effect of polymer structure on mineralization. The effect of polymerization time for the HEMA copolymer particularly illustrates this. Since these polymers are all amorphous and the copolymers have a random sequences there is no reason to anticipate any regular structures on the surface.
J. Burdon et al. /Materials Science and Engineering C4 (1996) 133-137
Hydroxylated polymers, t.hat are highly swellable in water but not actually soluble, seem to be most efficient. We had expected that copolymers containing c&boxylic acid groups would be effective but they do not seem to be. One interpretation is that the swollen polymer excludes the polyacrylic acid growth inhibitor, due to osmotic effects. A second view is that the hydroxylated pol:vmer provides the right amount of binding for the calcium ions to lower the crystal surface energy, without binding strongly to the surface and inhibiting growth. The use of a polymeric growth inhibitor has certainly been shown to be effective with other similar polymer films [ 7,8]. Zhang and Gonsalves interpret this effect in terms of a modification of the surface chemistry of the polymer film. We considered the possibility that nucleation occurred in the solution and crystals adsorbed to the surface. The effect of allowing the solution to stand and then adding the film is that the crystals grow as if put into a freshly mixed solution. Hence nucleation in the polymer does seem to be the predominant effect. It is clear from the micrographs that crystals do grow on the polymer surface but that the initial growth occurs within or partly within the surface and that crystals later break through. The increase of crystal size with time reflects a growth rate of 40 nm/min, which is reasonable. The relatively uniform crystal size suggests early nucleation followed by growth rather than continuing nucleation. This is explicable in terms of solute depletion in the polymer film or near the surface, once a substantial number of crystals have started to grow. Since nucleation is far more sensitive to supersaturation, it will be suppressed while growth continues [ 4,101. The moving growth front seen in Fig. 3 contradicts these ideas since it suggests that areas further from the edge nucleate later. In fact it moves slowly in from the film edge and then travels rapidly across the remaining film. We believe this moving front reflects nucleation on the glass-polymer interface or within the films, coupled with thinner films near the edge. The strong contrast seen at low magnification in the SEM is due to crystals which have broken through the surface, which happens earlier when the film is thinner. Unfortunately the very small sample size and bulb geometry, needed for crystallization under conditions of constant solution composition, do not lend themselves to sectioning through the films. Some evidence for the thickness effect is that the shape of the white band near the edge does apparently reflect changes in film topography. Similar films cast from solution onto microscope slides were weighed and found to have a thickness of a few microns.
137
Given that crystal growth originates within the film, the nucleation site may be in the polymer itself, on adventitious dust particles in the film or at the polymer-glass interface. These experiments were done with small areas of very thin films so that minimal change in the solution conditions occurs. There must nonetheless be some local change as the polymer equilibrates with the solution. Different results would be expected using large amounts of thick films or on gels which have a pH or other ion concentrations different from those in the solution.
5. Conclusions A series of hydroxylated acrylate polymers have been shown to promote surface mineralization by calcium oxalate from aqueous solution. Crystals initially form within the polymer but break through onto the surface as they grow. The effect is very sensitive to polymer structure and apparently reflects some combination of swellability and calcium ion binding.
Acknowledgements We would like to thank the Army Research support of this work.
Office for
References [I] P.D. Calvert and S. Mann, Journal ofMaterials Science, 23 ( 1988) 3801. [2] P. Rieke, B. Tarasevich, L. Wood, M. Engelhard, D. Baer, G. Fryxell, C. John, D. Laken and M. Jaehnig, Longmuir, 10 (1994) 619.
[3] S. Mann, D.D. Archibald, J.M. Didymus, T. Douglas, B.R. Heywood, F.C. Meldrum and N.J.Reeves, Science, 261 (1993) 1286. [4] P. Calve& Materials Research Society Symposium Proceedings, 330 (1994) 79. [5] M. Oner and P. Calvert, Materials Science and Engineering C, 2 (1994) 93. [ 61 P. Calve& M. Oner, J. Burdon, P. Rieke and K. Farmer, in SPIE: Smart Structures andMaterials; Albuquerque, NM, 1993, p. 354. [ 71 SK. Zhang and K.E. Gonsalves, Journal of Applied Polymer Science, 56 (1995) 687. [8] M.A. Crenshaw, A. Linde and A. Lussi, in LA. Aksay, G.L. McVay and J.F. Wager (eds.), Atomic and Molecular Processing of Electronicand Ceramic Materials; Materials Research Society,
Pittsburgh, PA, 1987, p. 99. [9] G.G. Odian, Principles of Polymerization, 3rd edn., Wiley, New York, 1991. [ 101 P.D. Calvert, Materials Research Society Symposium Proceedings, 73 (1986) 79.